1. Field of the Invention
The present invention relates to a radiation source, a lithographic apparatus, a device manufacturing method and a device manufactured thereby.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. In a lithographic apparatus as described above a device for generating radiation or radiation source will be present.
In a lithographic apparatus the size of features that can be imaged onto a substrate is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to be able to image smaller features. While most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation of around 13 nm. Such radiation is termed extreme ultraviolet, also referred to as XUV or EUV, radiation. The abbreviation ‘XUV’ generally refers to the wavelength range from several tenths of a nanometer to several tens of nanometers, combining the soft x-ray and vacuum UV range, whereas the term ‘EUV’ is normally used in conjunction with lithography (EUVL) and refers to a radiation band from approximately 5 to 20 nm, i.e. part of the XUV range.
Two main types of XUV electromagnetic radiation sources or sources are currently being pursued, a laser-produced plasma (LPP) and a discharge-produced plasma (DPP). In an LPP source, one or more pulsed laser beams are typically focused on a jet of liquid or solid to create a plasma that emits the desired radiation. The jet is typically created by forcing a suitable material at high speed through a nozzle. Such a device is described in U.S. Pat. No. 6,002,744, which disloses an LPP EUV source including a vacuum chamber into which a jet of liquid is injected using a nozzle.
In general, LPP sources have several advantages compared to DPP sources. In LPP sources, the distances between the hot plasma and the source surfaces are relatively large, reducing damage to the source components and thus reducing debris production. The distances between the hot plasma and the source surfaces are relatively large, reducing the heating of these surfaces, which in turn reduces the need for cooling and reduces the amount of infra-red radiation emitted by the source. The relatively open geometry of the construction allows radiation to be collected over a wide range of angles, increasing the efficiency of the source.
In contrast, a DPP source generates plasma by a discharge in a substance, for example a gas or vapor, between an anode and a cathode, and may subsequently create a high-temperature discharge plasma by Ohmic heating caused by a pulsed current flowing through the plasma. In this case, the desired radiation is emitted by the high-temperature discharge plasma. Such a device is described in U.S. Patent Application Publication 2004/0105082 A1, published Jun. 3, 2004, in the name of the applicant. This application describes a radiation source providing radiation in the EUV range of the electromagnetic spectrum (i.e. of 5-20 nm wavelength). The radiation source includes several plasma discharge elements, and each element includes a cathode and an anode. During operation, the EUV radiation is generated by creating a pinch as described in FIGS. 5A to 5E of U.S. Patent Application Publication 2004/0105082 A1. The application discloses the triggering of the pinch using an electric potential and/or irradiating a laser beam on a suitable surface. The laser used has typically a lower power than the laser(s) used in an LPP source.
In general, however, DPP sources have several differences compared to LPP sources. In DPP sources, the efficiency of the source is higher, approximately 0.5% for a DPP compared to 0.05% for an LPP. DPP sources also have a lower cost and require fewer, less expensive part replacements.
An improved source which combines the characteristics of a DPP electromagnetic radiation source or source with many characteristics of an LPP source is described in U.S. Patent Application Publication 2006/0011864 A1, published Jan. 6, 2006 in the name of the applicant. Although this source may reduce the amount of contamination produced, it will still produce debris from the discharge substance and ions which may enter the rest of the system. An additional problem with this source is the difficulty in using a discharge substance in the liquid state—for example pumping, transport and filtering need to be performed at a temperature above the melting point of the substance. In some cases, such as when using tin or lithium, the temperature of the liquid circuit has to be maintained above 230° C. and 180° C. respectively which considerably increase the complexity and cost of the source, and reduces the overall efficiency.
When any DPP source is operated using a discharge substance, such as tin, the contamination created in the form of debris and/or ions is relatively difficult to stop by means known in the art, such as foil-traps and magnetic/electric fields. Chemically-aggressive hot melted metals, such as tin, cause faster corrosion of most technologically convenient constructing materials, such as tungsten and molybdenum. This poses a serious threat to the apparatus using the source, for example a lithographic projection apparatus. This threat becomes significantly larger when the sources are scaled up in size and/or power in an attempt to create more intense radiation to increase the throughput of such a lithographic apparatus.